Magnetic disc memory units are widely used in data processing systems because such units have high storage capacity and require relatively short time for read/write heads of a disc memory to access data stored at a given point on the disc in response to a disc memory address supplied to the memory unit. Magnetic discs employed in disc memory units carry data on circular concentric tracks, typically positioned on both sides of the disc. The tracks generally have a width of no more than a few dozen microns. To transduce, i.e., read and write, data on the disc, magnetic heads of the memory units are positioned adjacent opposite faces of the disc, at a distance of a few tenths of a micron.
The magnetic discs are rotatably driven by an electric motor of the disc memory unit at a constant rotational speed. Current memory units frequently include a limited number of discs, i.e., one or two discs, and have a relatively limited storage capacity, on the order of ten to several tens of millions of bytes. Each byte typically includes eight bits, i.e., a binary digit data unit having a value equal to one or zero, as indicated by a magnetic flux transition on a track of the disc. Typically, at least one disc is enclosed in a cartridge that is selectively inserted into and removed from a receptacle of the magnetic disc memory unit. A disc memory unit receptacle normally contains only a single removable cartridge. When a cartridge is removed from the receptacle, another cartridge is immediately inserted into the receptacle as soon as read and/or write operations have been completed on the disc in the initially inserted cartridge. Thus, plural removable cartridges, each containing at least one magnetic disc, are used with a single disc memory unit.
Certain disc memory units contain two magnetic discs, one of which remains permanently inside the disc memory unit; the other disc is contained in a cartridge that is selectively inserted and removed from the disc memory unit receptacle.
Cartridges containing removable magnetic discs have standarized shapes and dimensions, enabling them to be used interchangeably with magnetic disc units of different manufacturers. Thereby, the standarized cartridges are easily inserted into and removed from the receptacles of many different magnetic disc memory units. The cartridges are constructed to protect discs located therein from contamination by dust and other particulate matter during periods while the cartridges are not in use, i.e., while a particular cartridge is not in a magnetic disc memory unit receptacle. The prior art cartridges typically include structure for ventilating the disc while the cartridge is in the disc memory unit receptacle, thereby to insure that the disc remains extremely clean, to increase reliability of data read from the disc. Exemplary of such a cartridge which is selectively inserted into and removed from a receptacle of a magnetic disc memory unit is disclosed in U.S. Pat. No. 4,078,246, commonly owned with the present invention, and entitled "Container For A Magnetic Disc".
The cartridge disclosed in the U.S. Pat. No. 4,078,246 contains a rigid disc having a relatively small diameter. The cartridge is relatively flat, having a height less than one tenth the length or width thereof. The cartridge is inserted into a suitable disc memory unit receptacle. The cartridge includes a side wall having an opening which is normally closed while the cartridge is not in use, i.e., when the cartridge is not in the disc memory unit receptacle. When the cartridge is in the disc memory unit receptacle, magnetic read/write heads of the unit extend through the opening and are moveable relative to recording tracks on the disc. The cartridge includes a mechanism for coupling a hub carrying the disc to a rotary drive mechanism, usually an electric motor, of the disc memory unit.
The cartridge includes rigid, elastically deformable, opposed, generally parallel walls having a tendency to be outwardly convex in a free state. At the center of one of the parallel walls is an internal projection, forming an annular bearing surface facing an inside face of the one wall. The other wall includes an opening coaxial with the center of the one wall. The opening in the other wall is surrounded by an annular bearing surface on the outside face of the other wall. The disc is carried by an annular hub having internal and external flanges positioned to engage the bearing surfaces of the other wall. The axial spacing between the flanges is less than the spacing between the bearing surfaces of the other wall while the other wall is in the free state, whereby when the disc is in an idle state and the parallel walls are flattened an elastic restoring force mechanically holds the disc in proper position. When the cartridge is in the receptacle of the magnetic disc memory and the disc is being driven, the disc is able to turn because the walls are urged toward each other, with a concave configuration. The bearing surfaces and flanges are constructed so that the cartridge is sealed when not in use, thereby protecting the hub carrying the disc and the disc from dust particles. By selecting suitable relative positions and diameters for the flanges, the disc is automatically ventilated during operation in response to a centrifugal effect of air on opposite faces of the disc, to provide cleaning of dust and other particular matter from the disc surface, as well as cooling.
As disclosed, for example, in commonly owned U.S. Pat. No. 4,298,898 data written onto discs enclosed in removable cartridges are separated into adjacent, circular, equal sized segments, with each side or face of the disc being normally divided into several dozen segments. Each segment is divided into two portions of different sizes, such that the larger portion contains data processed or to be processed by a data processing system including the disc memory unit containing the cartridge. The smaller portion contains track identification data used by the data processing system for positioning the read/write magnetic heads of the memory unit relative to the disc tracks. Within each segment, the smaller portion is separated into a number of reference areas. The number of reference areas is equal to the number of tracks, such that each track is associated with a single, separate area. The number of data bits per unit length along the circumference of a disc track is referred to as "longitudinal data density", while "radial density" indicates the number of tracks per unit length measured along the disc diameter.
The current trend in developing magnetic discs is focused particularly on obtaining substantial increases in radial and longitudinal densities. Typically, radial densities are on the order of 350 to 400 tracks per centimeter, i.e., 850 to 1,000 tracks per inch (TPI), while the longitudinal densities are on the order of 2,000 bits per centimer, i.e., 5,000 bits per inch (bpi).
It is difficult to obtain the same longitudinal and radial densities on removable magnetic discs enclosed in cartridges as on a disc which remains fixed permanently inside a disc memory unit. The removable feature of such cartridge enclosed magnetic discs is a limiting factor on the longitudinal and radial densities thereof. Different discs have differing mechanical dispersions, such that each disc has a different warp. In addition, different discs are located at slightly different center points within each cartridge relative to an axis of a drive motor. Thereby, variations subsist in the position of a disc track and face below magnetic heads of the disc memory unit. These variations are reflected in the amplitude of the electric signal derived by the read/write heads during a read operation, and can result in an error of a detected binary bit.
An additional limiting factor on the potential for obtaining high radial track densities, i.e., TPI, is that a coupling between a hub carrying the disc and a drive motor spindle of the disc memory unit may cause a particular track to be out of round. If a particular track is out of round, disturbances exist while track identification data are written into the reference areas and during read operations. The out of round track position results from assembly tolerances between the disc hub and the drive motor axis. The out of round position problem is further exacerbated by foreign particulate matter lodging in the various couplings between the cartridge and drive motor spindle.
To obtain the same high densities, in both the longitudinal and radial directions, for removable discs as is obtained for fixed discs, it is necessary to provide improved mechanical coupling tolerances between the disc tracks and unit heads, particularly by minimizing the out of round track positioning tolerances. It is desired to reduce the out of round track positioning tolerance to three to four microns, an improvement which is costly and difficult to obtain with the prior art cartridges, such as described in U.S. Pat. No. 4,078,246.
It is therefore an object of the present invention to provide a magnetic disc cartridge having improved mechanical coupling tolerances between the magnetic disc in the cartridge and magnetic head means of a magnetic disc memory unit.
Another object of the present invention is to provide a new and improved cartridge for at least one removable magnetic disc wherein out of round track positions caused by assembly tolerances between a hub carrying the disc and a drive motor axis for the hub are minimized.
The out of round condition has a tendency to exist in prior art cartridges containing removable magnetic discs because the disc axis of rotation is independent of the cartridge containing the disc, but is determined by the axis of the drive motor. Because the cartridge is not invariably located at the same position within the memory unit receptacle each time the cartridge is inserted in the receptacle, there is a shift in the disc position, thereby tending to increase the out of round condition between a disc track and disc memory unit head.